Description of the Prior Art
In a system of synchronous communication between data processing equipments one of the most widely used methods for sending messages consists in defining a frame structure characterized by:
a synchronization flag, PA1 special coding of the data to be transmitted so that the synchronization flag cannot be recognized in the middle of the data stream transmitted.
One of the best known ways to implement this method is to choose a flag which is a constant stream of P binary zeros followed by a binary 1. The data is then coded simply by inserting a binary 1 each time that a series of (P-1) binary 0 has been transmitted. For example, if the flag is `00001`, the message `0010 0000 10` is transmitted in the form: `00001 0010 00100 10`. The six underlined digits represent the synchronization flag and the inserted binary 1 (the spaces are included only to facilitate reading).
This method has a drawback: the time to transmit a message depends on its contents, which is a serious problem if a fixed routing time is required.
The known solution to this problem is to insert a binary 1 every (P-1) data bits transmitted: it is then certain that P consecutive binary zeros will never be encountered and the transmission time is always the same, regardless of the data transmitted. A well-known example of this method is the use of V.110 frames as defined by the CCITT (Comite Consultatif International du Telephone et du Telegraphe). These frames comprise a flag made up of eight binary zeros followed by a binary 1, a binary 1 being then inserted every seven bits to form a frame of 80 bits, 17 bits used for synchronization and 63 bits for the data.
A frame can be represented by a table with P columns and L rows. The first row, known as the locking row, includes P binary zeros and subsequent rows, known as data rows, each comprise a synchronization bit at binary 1 followed by (P-1) data bits.
The raw data signaling rate is usually defined as the number of bits of the frame transmitted per unit time and is therefore proportional to P.L, where L represents the number of rows of the half-rate frame. The usable data signaling rate is defined as the number of data bits of the frame transmitted during the same unit time and is therefore proportional to (P-1).(L-1). The transmission efficiency is the ratio of the usable data signaling rate to the raw data signaling rate, that is: EQU (P-1).(L-1)/P.L
It is thus clear that the efficiency will be proportional to the frame length, i.e. to the product of the number of columns by the number of rows P.L.
This frame structure is used to convey messages between a first equipment or transmitter and a second equipment or receiver. The receiver is activated at a random time with the result that the time to recognize the start of a message, i.e. the locking row, is dependent on the length of the frame; the recognition time is equal to half the duration of a frame, on average.
It is therefore clear that to increase transmission efficiency it is necessary to increase the length of the frame, which has the effect of increasing the recognition time. This is not desirable because the recognition time conditions all of the transmission procedure, including synchronization of the receiver. If the receiver must also send messages to the transmitter, it usually does so only after synchronization has been achieved.
The known solution is to start transmission by sending special synchronization frames. These are very short frames whose function is as follows: the transmitter begins to send these frames and waits for a response from the receiver. As soon as the receiver begins to receive these synchronization frames it sends a normal message as an acknowledgement of the synchronization frame. The transmitter can then start to send long frames, the receiver being ready to receive them with no time-delay.
Apart from being complicated to implement, this method has the drawback that synchronization takes time to establish: the synchronization flag must be transmitted in full, received and interpreted by the receiver before it responds. The transmitter must then switch to normal communication mode, which may not happen immediately if it must simply retransmit messages which reach it synchronized, since in this case it must wait for the next received message before it can retransmit it. For example, if it receives messages at times T, 2T, . . . , nT, arrival of the acknowledgement between T and 2T will impose a wait until 2T before retransmission can start: it is clear that the synchronization recognition time-delay can lead to a much greater time-delay overall.
This solution is therefore unsatisfactory.
An object of the present invention is a frame structure which can convey messages with good transmission efficiency combined with a message recognition time within acceptable limits.